EXCEED THE SPACE PROVIDED. We propose developing a sector at the Advanced Photon Source for crystallographic studies equiring undulator radiation. The sector will consist of two undulator stations and one bending magnet station, optimized for X-ray diffraction studies of crystals with large unit cells, small or weakly diffracting crystals, new MAD phasing methods, structures that are limited by signal-to- noise considerations and other projects that challenge the capabilities of existing synchrotron beamlines. A dual undulator configuration in which one undulator provides radiation to a fully unable end station and the second undulatorprovides radiation to a tunable side station is planned. We will pursue a technological R&D program that consists of five areas: (1) sector design and construction, including development of a dual hard X-ray undulatorconfiguration and X-ray optics, 2) X-ray instrumentation, includingX-ray detectors and precision goniometers, (3) optimization of sample, including cryocystallography, elimination of radiation damage, microcrystals and large unit cells, (4) new MAD phasing methods and (5) computational tools, including data management, web-based experiments, GUIs and integrated systems for analysis. Core and collaborative research projects will be provided by scientists from Columbia, Cornell; Harvard, Rockefeller and Yale Jniversities along with the Memorial Sloan-Kettering Cancer Center. These scientists work on a variety of technically challenging projects and will drive the development of the facility. The cientific program focuses broadly on cancer biology, immunology, virology and neurobiology as well as the basic disciplines of biochemistry, cell biology, molecular biology and biophysics. Individual projects emphasize signal transduction, DNA transcription and regulation, cell cycle regulation, virus structure and function, membrane proteins, protein folding and enzyme structure and function. The resource will operate a user program that makes the undulatorresource available o the general crystallographic community.